Optical encoders are essential to many position-sensing devices used today to detect both linear motion and rotary motion. Typically, an optical encoder is comprised of two major subassemblies: a code disc and a sensing element. The code disc for linear optical encoders is frequently called a code strip. Most optical encoders, however, are rotary optical encoders that utilize a flat wheel having transparent windows as the code disc. The transparent windows are usually perforations that allow for the passage of light, the perforations being aligned in patterns around a central axis. The sensing elements of most rotary optical encoders are photodetectors that detect a light source when it passes through the transparent windows of a code disc. Very commonly, the light source is a light emitting diode (LED) because of the low cost and nearly monochromatic quality of the emitted light. When the code disc is rotated about its central axis, such as when it is attached to the shaft of a motor, the code disc blocks the light source unless a transparent window allows for the passage of light. Each time the light source passes through a moving transparent window, a pulse of light emerges, but is cut off when the code disc abruptly blocks the light source. Photodetectors count the number of light pulses that are generated by the rotating code disc, thereby determining the position of a particular object. In the example of a motor shaft, the rotary optical encoder is capable of precisely determining the number of rotations of the motor.
The light source in an optical encoder must be collimated so that the emitted light can be uniformly directed at the sensing element. A collimating lens that is external to the light source most commonly performs collimation of the light source. The collimating lens's shape is determined by the relative indices of refraction between the lens material and the surrounding medium. Until now, optical encoders have been designed for operation in air, so the lens's shape assumes that the medium surrounding the lens will be air. For applications where the position of a device in a fluid must be determined, optical encoders are equipped with seals that prevent any liquids from entering a fully encased optical encoder.
Shaft seals, designed to protect against contamination and moisture in the environment, are currently viewed as necessary when an optical encoder needs to be protected against contamination from liquids. If optical encoders are not sealed, there is a significant degradation of performance when liquids disrupt the optical components. O-ring seals are also highly recommended for all points of entry were fluids might seep into the encoder, usually being required around screws that hold the optical encoder's case together and properly mounted. In addition to the expense of sealing such an optical encoder, shaft seals impose additional load on the motor, or other force, that ultimately drives the code disc. If any one seal breaks, the optical encoder will stop working properly. The method of the present invention is a more reliable approach that significantly cuts costs and that does not add any drag to the motor.